But the biggest "investor" in US government debt is the Fed, so I don't quite follow how you can separate the two things here. And the other investors are using

But the biggest "investor" in US government debt is the Fed, so I don't quite follow how you can separate the two things here. And the other investors are using money that was ultimately issued into existence via lending against reserves, and the Fed controls interest rates by issuing or withdrawing high powered money. And the money the government spends after borrowing it is re-deposited back into the banking system, raising their reserves and letting them issue more loans, further increasing the size of the monetary base and therefore inflation. Given that it's all connected I don't quite follow why you are cleanly dividing them here.